HTML & Email Structure

html5-semantics

CSS property · 7 supported, 8 partial, 5 unsupported across 21 clients

As of Oct 14, 2026, html5-semantics fails or degrades in 14 of 21 email clients, treat it as progressive enhancement, never structure.

2/7 webmail, 3/6 desktop, 2/8 mobile clients support html5-semantics fully.
Unsupported in Outlook Classic, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS and 1 more.
Partial in Gmail, Gmail Android, Gmail iOS, Outlook 365 and 4 more, behavior differs from browsers, test before relying on it.
Outlook Classic (Word engine, supported to at least 2029) does not fully support html5-semantics, and its replacement changes little here; see the deprecation diff.
Across all clients, only 6 of 298 tracked HTML/CSS features render identically everywhere.

Source: caniemail.com (MIT), via the Emailens engine. Last tested upstream Jul 29, 2019, synced by Emailens Oct 14, 2026.

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Partial^{See caveat 1}
Gmail Android	mobile	Gmail Mobile	Partial^{See caveat 1}
Gmail iOS	mobile	Gmail Mobile	Partial^{See caveat 1}
Outlook 365	webmail	Outlook Web	Partial^{See caveat 2}
Outlook (New)	desktop	Outlook Web	Partial^{See caveat 2}
Outlook Classic	desktop	Microsoft Word	Unsupported
Outlook iOS	mobile	Outlook Mobile	Partial^{See caveat 2}
Outlook Android	mobile	Outlook Mobile	Partial^{See caveat 2}
Outlook for Mac	desktop	WebKit	Supported
Apple Mail	desktop	WebKit	Supported
Apple Mail iOS	mobile	WebKit	Supported
Yahoo Mail	webmail	Yahoo	Unsupported^{See caveat 3}
Yahoo Mail Android	mobile	Yahoo	Unsupported^{See caveat 3}
Yahoo Mail iOS	mobile	Yahoo	Unsupported^{See caveat 3}
Samsung Mail	mobile	Samsung	Supported
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Partial^{See caveat 4}
Proton Mail	webmail	Proton	Supported
AOL Mail	webmail	AOL	Unsupported^{See caveat 3}
Fastmail	webmail	Fastmail	Supported
Superhuman	desktop	Blink	Unknown

Documented caveats (4)

1.Gmail, Gmail Android, Gmail iOS: Partial. <details>, <main>, <nav> and <summary> are not supported. Opening and closing tags are replaced by <u></u>.
2.Outlook 365, Outlook (New), Outlook iOS, Outlook Android: Partial. Only <time> is supported. Unsupported opening and closing tags are stripped.
3.Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail: Unsupported opening and closing tags are stripped.
4.HEY Mail: Partial. <article>, <aside>, <header>, <main> <nav>, <section>, <summary> are not supported.

Caveats recorded by caniemail.com (MIT) and carried verbatim, one entry per wording rather than per client.

Client-by-client behaviour for html5-semantics

Fully supports html5-semantics (7): Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird, Proton Mail, Fastmail.

Partial support (8): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook (New), Outlook iOS, Outlook Android, HEY Mail. Expect rendering quirks unique to each engine. Partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

No support (5): Outlook Classic, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail. Plan fallbacks for these clients before relying on html5-semantics in production sends.

Behaviour unverified in: Superhuman.